The present invention relates generally to conversion of solar energy to heat, and more specifically to a new class of solar selective absorption coatings.
Solar selective absorption coatings applied to solar absorber components have considerable utility in the design of solar thermal flat-plate collectors and of solar concentrators. Such coatings significantly improve the thermal conversion efficiency of such units by reducing radiative energy losses from the absorbing elements.
A measure of the thermal effectiveness of a solar selective absorption coating is the ratio of its absorption of solar energy, A, and its thermal emission at the system operating temperature, E. Solar selective absorption coatings having A/E ratios of about 5 or greater extend the practical geographic range of solar thermal systems into marginal climatic locations, while also allowing systems to operate at higher temperatures, resulting in improved efficiency and lower operating costs.
A wide range of solar selective absorption coatings have been used to address these needs. These include black organic coatings, various types of structured metallic coatings, and cermet coatings. These coatings share a common set of problems which limit their range of application. All such coatings experience degradation owing to the elevated system operating temperature, requiring a trade-off between the improved conversion efficiency associated with high operating temperatures and insuring that the coating have a practical operating lifetime. Additionally, such coatings tend to be mechanically fragile, leading to manufacturing and maintenance difficulties. There is therefore a need for improved solar selective absorption coatings which address these limitations. An additional need is for improved solar selective absorption coatings which can be fabricated economically, and with less impact on the environment.
The present invention is of a new class of solar selective absorber coatings. These coatings include a surface which has low emissivity at the system operating temperature, said surface to be coated by a nanostructured selective absorption layer which has high solar absorptivity. The composite surface is then coated by a sol-gel protective layer. The invention also includes a process for depositing such a solar selective absorption coating on a substrate. The new process is economical, energy efficient, and has limited environmental impact compared to competing processes.